This download looks like a map. You need to place it in your maps folder inside the Garry's Mod folder.
You should end up with a directory structure similar to below
Don't know where the Garry's Mod folder is or how to extract zip files? See more useful installation tips.
Gaster Blaster, 娜塔莎, jsrivera10, patrick-ed, Private_Baldrick, re15revisited, 全杀机最骚的汪涵, Nekko, 失落園, , [TW]Mapleleaf, Jacket, tim360226, [XY]王妞妞, dcorrea566, Lose some weight fatass., noname0949, Traxtorm | AniMe, BenAckerley, omtan51, toylove, minjintheniqqawhoalways, Gibuya, Pz.IVAusf.H, MAX, Traud, Bear, nc15doritos, mrhin1993, Lazy Ghost, 新北永和 金城武, Trager, 狼玥, inclueD, Baş Çaycı Hüseyin, Vikings North, Nevermore, Foxypaws have Foxyfoofs, Shadow凌, A-Mod, jdhr24, FlamingMojo, [TS] TGGreen, The Great White North, KITKWONG, whuppo, yavkidegamer, princecloud0522, VooDoo, VooDoo +11 others